IntermolecularForces49A crystalline solid possesses rigid and long-range order. In acrystalline solid, atoms, molecules or ions occupy specific(predictable) positions.An amorphous solid does not possess a well-definedarrangement and long-range molecular order.A unit cell is the basic repeating structural unit of a crystallinesolid.latticepointUnit Cell Unit cells in 3 dimensionsAt lattice points:• Atoms• Molecules• Ions
IntermolecularForces61When silver crystallizes, it forms face-centered cubic cells. Theunit cell edge length is 409 pm. Calculate the density of silver.d =mVV = a3= (409 pm)3= 6.83 x 10-23cm34 atoms/unit cell in a face-centered cubic cellm = 4 Ag atoms107.9 gmole Agx1 mole Ag6.022 x 1023atomsx = 7.17 x 10-22gd =mV7.17 x 10-22g6.83 x 10-23cm3= = 10.5 g/cm3
IntermolecularForces62An Arrangement for Obtaining the X-ray Diffraction Patternof a Crystal.
IntermolecularForces63Extra distance = BC + CD = 2d sinθ = nλ (Bragg Equation)Reflection of X rays from Two Layers of Atoms.
IntermolecularForces64X rays of wavelength 0.154 nm are diffracted from a crystal at anangle of 14.17o. Assuming that n = 1, what is the distance (in pm)between layers in the crystal?nλ = 2d sin θ n = 1 θ = 14.17oλ = 0.154 nm = 154 pmd =nλ2sinθ=1 x 154 pm2 x sin14.17= 314.0 pm
IntermolecularForces65Types of CrystalsIonic Crystals• Lattice points occupied by cations and anions• Held together by electrostatic attraction• Hard, brittle, high melting point• Poor conductor of heat and electricityCsCl ZnS CaF2
IntermolecularForces66Types of CrystalsCovalent Crystals• Lattice points occupied by atoms• Held together by covalent bonds• Hard, high melting point• Poor conductor of heat and electricitydiamond graphitecarbonatoms
IntermolecularForces67Types of CrystalsMolecular Crystals• Lattice points occupied by molecules• Held together by intermolecular forces• Soft, low melting point• Poor conductor of heat and electricitywater benzene
IntermolecularForces68Types of CrystalsMetallic Crystals• Lattice points occupied by metal atoms• Held together by metallic bonds• Soft to hard, low to high melting point• Good conductors of heat and electricityCross Section of a Metallic Crystalnucleus &inner shell e-mobile “sea”of e-
IntermolecularForces72An amorphous solid does not possess a well-definedarrangement and long-range molecular order.A glass is an optically transparent fusion product of inorganicmaterials that has cooled to a rigid state without crystallizingCrystallinequartz (SiO2)Non-crystallinequartz glass